| Summary: | Based on the meteorological, ozone (O<sub>3</sub>), and vertical observation data of 2020, this study sought to evaluate the daily variation in O<sub>3</sub>, particularly the characteristics of nocturnal ozone pollution. We also discuss the effect of local and mesoscale horizontal transport and vertical mixing on the formation of nocturnal O<sub>3</sub> pollution. Distinct seasonal characteristics of the daily O<sub>3</sub> variation in Shenzhen were identified. In particular, significant nocturnal peaks were found to regularly occur in the winter and spring (November–December and January–April). The monthly average of daily variation had a clear bimodal distribution. During the period, O<sub>3</sub> pollution frequently occurred at night, with the maximum hourly O<sub>3</sub> concentration reaching 203.5 μg/m<sup>3</sup>. Nocturnal O<sub>3</sub> pollution was closely associated with horizontal transport and vertical mixing. During the study period, the O<sub>3</sub> maximum values were recorded on 68 nights, primarily between 23:00 and 03:00, with occasional observation of two peaks. The impact of horizontal transport and vertical mixing on the nocturnal secondary O<sub>3</sub> maximum values was elaborated in two case studies, where vertical mixing was mainly associated with low-level jets, with strong wind shear enhancing turbulent mixing and transporting O<sub>3</sub> from the upper layers to the surface.
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